Method and apparatus of exposure for correcting electrostatic latent image outline

ABSTRACT

An exposure method for use in an image forming apparatus for having each of edge emission type electroluminescent (EL) devices emit light a plurality of times to form one pixel of an image. The gradient of the image outline relative to the horizontal scanning direction is checked to see if it falls in one of two ranges: between 0° and 45°, or between 45° and 90°. If the gradient falls between 0° and 45°, the edge emission type EL devices are made to emit light selectively according to the proportion of image-filled regions in the pixels intersected by the outline. If the gradient falls between 45° and 90°, either the light intensity of the edge emission type EL devices is established according to the proportion of image-filled regions in the pixels intersected by the outline, or the number of the pulses which exceed a threshold voltage and which are fed to the edge emission type EL devices is gradually reduced according to reductions of image-filled regions in the pixels intersected by the outline.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a method and apparatus of exposure forforming electrostatic latent images on a photosensitive body byelectrophotography and, more particularly, to a method and apparatus ofexposure for correcting the outline of electrostatic latent images.

Recent years has seen the advent of a new type of line printer. Theytypically have a line head positioned opposite to a photosensitive body,the head comprising a large number of edge emission typeelectroluminescent (EL) devices arranged in the horizontal scanningdirection. The outer circumference of the photosensitive body iselectrically charged. When a group of edge emission type EL devices ofthe line head emits light to the charged portion, an electrostaticlatent image is formed over the photosensitive body. The electrostaticlatent image is developed and transferred onto a sheet of paper. Each ofthe edge emission type EL devices on the line head is made of a thinfilm active layer enclosed by a dielectric substance, the thin filmactive layer containing zinc sulfide and other active elements, bothsides of the dielectric substance having electrodes. This type of ELdevice is quicker in responsiveness and far more enhanced in lightintensity than conventional light emitting devices. FIG. 13 shows partof an image obtained conventionally based on print data. In the figure,x stands for the size of one pixel in the horizontal scanning direction;and y, for the size of one pixel in the vertical scanning direction. Thewidth of an edge emission type EL device in the horizontal scanningdirection corresponds to x, and the thickness of the device matches oneof a plurality of parts making up its width. Thus, when the drum-shapedphotosensitive body rotates, one edge emission type EL device emitslight a plurality of times to form one pixel of an electrostatic latentimage.

As depicted in FIG. 13, the ideal outline L of the image is inclinedrelative to the horizontal scanning direction depending on the printdata. As long as the image is constituted in units of pixels, it isimpossible to approximate the outline to a straight line; the outline ofthe electrostatic latent image is bound to be jagged.

One conventional way to minimize the jaggedness of the outline is asfollows: Because a light emission from each edge emission type EL deviceforms a compressed rectangle that is horizontally long and verticallyshort, edge emission type EL devices are made to emit light selectivelydepending on how each fringe pixel of the image is intersected by theoutline. Meanwhile, one pixel is formed by a plurality of emissions, andone emission forms one of a plurality of components constituting onepixel. Thus each pixel is made of a plurality of flat-shaped pixelcomponents arranged vertically. When the image is formed not in units ofpixels but in units of pixel components, the jaggedness of the outlineis appreciably reduced.

The above prior art method is effective when the gradient of the outlineL is small relative to the horizontal scanning direction. This isbecause the emission from each edge emission type EL device forms acompressed rectangle that is vertically short. However, as illustratedin FIG. 14, when the gradient of the outline L relative to thehorizontal scanning direction exceeds 45° and becomes closer to 90°,setting the number of emissions in units of pixel components stillleaves a significant amount of image-filled regions outside the outlineL. Inside the outline L, there grow blank regions within the image.These characteristics pronounce the jaggedness of the outline, notdiminish it.

OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anexposure method for forming an electrostatic latent image with anoutline of reduced jaggedness.

It is another object of the invention to provide an exposure apparatuscapable of implementing the inventive exposure method.

In carrying out the invention and according to one aspect thereof, thereis provided an exposure method for use in an image forming apparatushaving a large number of edge emission type electroluminescent (EL)devices, the horizontal width of each of the devices being greater thanthe vertical thickness thereof, the devices being arranged in thehorizontal scanning direction and positioned opposite to aphotosensitive body so that as the photosensitive body rotates, one edgeemission type EL device emits light a plurality of times onto thesurface of the body to form one pixel of an electrostatic latent image;the method comprising the steps of: determining one of a first and asecond case of print data, the first case being one in which thegradient of the outline of the electrostatic latent image relative tothe horizontal scanning direction falls between 0° and 45°, the secondcase being one in which the gradient of the same outline relative to thehorizontal scanning direction falls between 45° and 90°; causing, in thefirst case, the edge emission type EL devices t emit light selectivelyaccording to the proportion of image-filled regions in the pixelsintersected by the outline; setting, in the second case, the lightintensity of the edge emission type EL devices according to theproportion of image-filled regions in the pixels intersected by theoutline; and gradually reducing, alternatively in the second case, thenumber of pulses applied to the edge emission type E devices accordingto reductions of image-filled regions in the pixels intersected by theoutline, the pulses exceeding a predetermined threshold voltage value.

When the gradient of the image outline relative to the horizontalscanning direction is judged to be smaller than 45°, the edge emissiontype EL devices are made to emit light selectively according to theproportion of image-filled regions in the pixels intersected by theoutline. This feature takes advantage of the compressed rectangle shapeof each edge emission type EL device in minimizing the image-filledregions extending outside of the outline and the blank regions insidethereof. When the regions flanking the outline are minimized, thejaggedness of the outline is made less conspicuous. When the gradient ofthe image outline relative to the horizontal scanning direction isjudged to be greater than 45°, there are two options. Either the lightintensity of the edge emission type EL devices is reduced according tothe proportion of image-filled regions in the pixels intersected by theoutline; or the number of pulses applied to the edge emission type ELdevices is gradually reduced according to reductions of image-filledregions in the pixels intersected by the outline, the pulses exceeding apredetermined threshold voltage value. This lowers the luminous densityof the image portions extending outside the outline and thereby makesthe jaggedness of the outline less pronounced visually. Further objects,features and advantages of the invention will become more apparent uponreading the following description and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a) is an electronic circuit diagram of a first embodiment of theinvention;

FIG. 1 (b) is a schematic view showing how a line head is positionedrelative to a photosensitive body in the first embodiment of FIG. 1 (a);

FIG. 2 is a flowchart of steps in which the first embodiment of FIG. 1(a) works;

FIG. 3 is a schematic view of part of an image printed in the firstembodiment using print data;

FIG. 4 is a schematic view of part of another image printed in the firstembodiment using print data;

FIGS. 5(a) and 5(b) are a set of timing charts showing voltage pulsesapplied to common electrodes and to channel electrodes in the firstembodiment;

FIGS. 6(a) and 6(b) are a set of timing charts depicting therelationships between light intensity and pulses applied to edgeemission type EL devices in the first embodiment;

FIG. 7 is an electronic circuit diagram of a second embodiment of theinvention;

FIG. 8 is a flowchart of steps in which the second embodiment works;

FIG. 9 is a schematic view of part of an image printed in the secondembodiment using print data;

FIG. 10 is a schematic view of part of another image printed in thesecond embodiment using print data;

FIG. 11 is a timing chart indicating how the number of pulses is changeddepending on a reduced proportion of the image-filled region in onepixel, the pulses being applied to one edge emission type EL device inthe second embodiment;

FIG. 12 is a view showing the image of one pixel which is formed after achange of a pulse count;

FIG. 13 is a view depicting part of an image formed conventionally basedon print data; and

FIG. 14 is a view sketching part of another image formed conventionallybased on print data.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The first embodiment of the invention will now be described withreference to FIGS. 1 through 6. FIG. 1(a) is an electronic circuitdiagram of an exposure apparatus practiced as the first embodiment ofthe invention. In the first embodiment, as illustrated, a host computer1, a data decision circuit block 2 and a data conversion circuit 3 areconnected in series. The host computer 1 is an external input unit, andthe data decision circuit block 2 is data determining means thatconstitutes part of the apparatus. Print data transferred from the hostcomputer 1 to the data decision circuit block 2 are bit map-based oroutline font-based print data. In any case, the data about the gradientof an image outline relative to the horizontal scanning direction isincluded in the print data. This allows the data decision circuit block2 receiving the print data to determine one of two ranges in which thegradient of the outline falls: one between 0° and 45°, the other between45° and 90°. A data decision signal resulting from the abovedetermination is output together with the print data to the dataconversion circuit 3. The data conversion circuit comprises lightemission determining means 4 and light intensity setting means 5. Whenthe gradient of the outline falls between 0° and 45°, the light emissiondetermining means 4 determines the light emitting action of edgeemission type EL devices (to be described later) according to theproportion of image-filled regions in the pixels intersected by theoutline. When the gradient of the outline falls between 45° and 90°, thelight intensity setting mean 5 sets the light intensity of edge emissiontype EL devices according to the proportion of image-filled regions inthe pixels intersected by the outline. A head control circuit 6, alsoincluded in the first embodiment, forwards a driving signal from thelight emission determining means 4 and another driving signal from thelight intensity setting means 5 to a common driver 7 and a channeldriver 8, respectively. A line head 9 comprises a large number of edgeemission type EL devices 10 arranged in the horizontal scanningdirection on a substrate 25, as shown in FIG. 1 (b). These devices 10are each made of a thin film active layer enclosed by a dielectricsubstance, the thin film active layer containing zinc sulfide and otheractive elements, both sides of the dielectric substance having commonelectrodes 11 and channel electrodes 12. The common electrodes 11 areconnected to the common driver 7, and the channel electrodes 12 to thechannel driver 8.

As shown in FIG. 1(b), the line head 9 is positioned opposite to aphotosensitive body 27. Around the photosensitive body 27 are positioneda charging unit 29, a developing unit 31, a transfer unit 33 and acleaning unit 35 of the known type each. These units operate inconventional ways, and descriptions of their workings are thus omitted.

In operation, each channel electrode 12 is supplied with low-voltagepulses V₁ of a positive-negative symmetrical amplitude, and each commonelectrode 11 is fed with high-voltage pulses V₂ of also apositive-negative symmetrical amplitude, as illustrated in FIG. 5. Thehigh-voltage pulses V₂ are close to a threshold voltage ±V_(TH).Application of the pulses causes a pulse voltage difference V₂ -V₁ to besent to the edge emission type EL device 10. When the pulses V₁ and V₂are controlled appropriately in synchronism, an emission state of FIG. 5(a) and a non-emission state of FIG. 5(b) are obtained.

Thus as shown in FIG. 6, the edge emission type EL device 10 emits lightwhen fed with pulses exceeding the threshold value of ±V_(TH) and havinga positive-negative symmetrical amplitude. The light intensity of eachemission gradually increases, then peaks off followed by an afterglow.FIG. 6(a) illustrates how an average light intensity D remains highbecause of a short emission peak-to-peak cycle when the edge emissiontype EL device 10 is fed continuously with pulses exceeding thethreshold value of ±V_(TH). FIG. 6(b), on the other hand, depicts how anaverage light intensity B remains lower than D because of a longeremission peak-to-peak cycle when the edge emission type EL device 10 isfed with pulses exceeding the threshold value of ±V_(TH) every threepeaks. The latter average light intensity B is so controlled as to occurat any of four levels depending on the application frequency of pulsesexceeding the threshold value of ±V_(TH). The four levels are:non-emission (A), weak emission (B), medium emission (C) and strongemission (D). The light intensity is necessarily proportional to thevoltage value of the electrostatic latent image formed on thephotosensitive body.

How the exposure in the first embodiment actually takes place will nowbe described with reference to the flowchart of FIG. 2. The apparatusinitially enters step ST1 after getting started. In step ST1, based onthe print data sent from the host computer 1, the data decision circuitblock 2 determines one of two ranges that the gradient of the imageoutline relative to the horizontal scanning direction falls in: onebetween 0° and 45°, the other between 45° and 90°. The result of thedetermination (i.e., a data decision signal) is output together with theprint data to the data conversion circuit 3. If the gradient of theimage outline relative to the horizontal scanning direction is judged tofall between 0° and 45°, step ST2 is taken. In step ST2, the lightemission determining means 4 determines the light emitting action ofedge emission type EL devices 10 according to the proportion ofimage-filled regions in the pixels intersected by the outline. If, instep ST1, the gradient of the image outline relative to the horizontalscanning direction is judged to fall between 45° and 90°, step ST3 istaken. In step ST3, the light intensity setting means 5 sets the lightintensity of edge emission type EL devices 10 according to theproportion of image-filled regions in the pixels intersected by theoutline.

FIGS. 3 and 4 sketch parts of images obtained by the first embodimentusing print data. These are examples that contain outlines L havinggradients relative to the horizontal scanning direction. FIG. 3 shows anexample in which the gradient of the outline L relative to thehorizontal scanning direction is less than 45. In this case, edgeemission type EL devices 10 are driven selectively based on the lightemission data from the light emission determining mean 4. This reducesthe number of pixel components in the image-filled regions of the pixelsintersected by the outline, the reduction being achieved depending onthe proportion of such regions in those pixels. As a result, theimage-filled regions extending outside the outline L are minimized, andthe jaggedness of the outline L becomes much less pronounced.

FIG. 4 shows an example in which the gradient of the outline L relativeto the horizontal scanning direction is greater than 45° and closer tothe vertical scanning direction. In this case, the frequency ofsupplying edge emission type EL devices 10 with pulses exceeding thethreshold value of ±V_(TH) is established on the basis of the lightintensity (A, B, C or D) set by the light intensity setting means 5.This lowers the luminous density of the image portions extending outsidethe outline L, thereby subduing the jaggedness of the outline visually.In FIG. 4, the concentration of oblique lines corresponds to the lightintensity level (A, B, C or D), i.e., the concentration is proportionalto the luminous density involved.

The second embodiment of the invention will now be described withreference to FIGS. 7 through 12. Both in the second embodiment and inthe first embodiment, like reference characters designate like orcorresponding parts. FIG. 7 is an electronic circuit diagram of anexposure apparatus practiced as the second embodiment of the invention.As illustrated, the second embodiment comprises a host computer 1, adata decision circuit block 2 and a data conversion circuit 3 connectedin series. The host computer 1 is an external input unit, and the datadecision circuit block 2 is data determining means that constitutes partof the apparatus. Print data transferred from the host computer 1 to thedata decision circuit block 2 are bit map-based or outline font-basedprint data. In any case, the data about the gradient of an image outlinerelative to the horizontal scanning direction is included in the printdata. This allows the data decision circuit block 2 receiving the printdata to determine one of two ranges in which the gradient of the outlinefalls: one between 0° and 45°, the other between 45° and 90°. A datadecision signal resulting from the above determination is outputtogether with the print data to the data conversion circuit 3. The dataconversion circuit 3 comprises light emission determining means 4 andpulse count changing means 20. When the gradient of the outline fallsbetween 0° and 45°, the light emission determining means 4 determinesthe light emitting action of edge emission type EL devices (to bedescribed later) according to the proportion of image filled regions inthe pixels intersected by the outline. When the gradient of the outlinefalls between 45° and 90°, the pulse count changing means 20 graduallyreduces the number of pulses which exceed a predetermined thresholdvoltage and which are fed to edge emission type EL devices 10. Thus, thepulse count reduction is achieved according to reductions of imagefilled regions in the pixels intersected by the outline. A head controlcircuit 6, also included in the second embodiment, forwards a drivingsignal from the light emission determining means 4 and another drivingsignal from the pulse count changing means 20 to a common driver 7 and achannel driver 8, respectively. A line head 9 comprises a large numberof edge emission type EL devices 10 arranged in the horizontal scanningdirection on a substrate, not shown. These devices 10 are each made of athin film active layer enclosed by a dielectric substance, the thin filmactive layer containing zinc sulfide and other active elements, bothsides of the dielectric substance having common electrodes 11 andchannel electrodes 12. The common electrodes 11 are connected to thecommon driver 7, and the channel electrodes 12 to the channel driver 8.

In operation, each channel electrode 12 is supplied with low-voltagepulses V₁ of a positive-negative symmetrical amplitude, and each commonelectrode 11 is fed with high-voltage pulses V₂ of also apositive-negative symmetrical amplitude, as illustrated in FIG. 5 inconnection with the first embodiment. The high voltage pulses V₂ areclose to a threshold voltage ±V_(TH). Application of the pulses causes apulse voltage difference V₂ -V₁ to be sent to the edge emission type ELdevice 10. When the pulses V₁ and V₂ are controlled appropriately insynchronism, an emission state of FIG. 5(a) and a non-emission state ofFIG. 5(b) are obtained.

How the exposure in the second embodiment actually takes place will nowbe described in more detail with reference to the flowchart of FIG. 8.In step ST10, based on the print data sent from the host computer 1, thedata decision circuit block 2 determines one of the two ranges that thegradient of the image outline relative to the horizontal scanningdirection falls in: one between 0° and 45°, the other between 45° and90°. The result of the determination (i.e., a data decision signal) isoutput together with the print data to the data conversion circuit 3. Ifthe gradient of the image outline relative to the horizontal scanningdirection is judged to fall between 0° and 45°, step ST20 is taken. Instep ST20, the light emission determining means 4 determines the lightemitting action of edge emission type EL devices 10 according to theproportion of image-filled regions in the pixels intersected by theoutline. If, in step ST10, the gradient of the image outline relative tothe horizontal scanning direction is judged to fall between 45° and 90°,step ST30 is taken. In step ST30, the pulse count changing means 20gradually reduces the number of pulses which exceed a predeterminedthreshold voltage and which are fed to edge emission type EL devices 10,the pulse count reduction being achieved according to reductions ofimage-filled regions in the pixels intersected by the outline.

FIGS. 9 and 10 sketch parts of images obtained by the second embodimentusing print data. These are examples that contain outlines L havinggradients relative to the horizontal scanning direction. FIG. 9 shows anexample in which the gradient of the outline L relative to thehorizontal scanning direction is less than 45°. In this case, edgeemission type EL devices 10 are driven selectively based on the lightemission data from the light emission determining means 4. This reducesthe number of pixel components in the image-filled regions of the pixelsintersected by the outline, the reduction being achieved depending onthe proportion of such regions. As a result, the image-filled regionsextending outside the outline L are minimized, and the jaggedness of theoutline L becomes much less pronounced.

FIG. 10 shows an example in which the gradient of the outline L relativeto the horizontal scanning direction is greater than 45° and closer tothe vertical scanning direction. In this case the edge emission type ELdevices 10 are fed with pulses according to the pulse count establishedby the pulse count changing means 20. FIG. 11 is a timing chart in whichthe horizontal direction represents the time in which each pixel isformed, and the vertical direction indicates the amplitude of the pulseswhich exceed the threshold value of ±V_(TH) and which are fed to theedge emission type EL devices 10. What FIG. 11 shows is how the pulsecount is gradually lowered as the proportion of image-filled regions inthe pixels intersected by the outline is getting reduced. FIG. 12sketches the image of one pixel formed by the pulse application depictedin FIG. 11. In FIG. 12, the shaded regions correspond to exposedportions (i.e., image-filled regions).

As the proportion of image-filled regions decreases in the pixelsintersected by the outline L, the edge emission type EL devices 10involved are made to emit light intermittently, as shown in FIG. 10.This provides a two-tone representation of a reduced image-filled regionin each of the pixels intersected by the outline. As a result, thejaggedness of the image outline is made less conspicuous visually.

When pulses exceeding a predetermined threshold voltage are appliedintermittently during formation of a single pixel, as depicted in FIG.11, each emission grows in intensity and peaks off followed by anafterglow. This arrangement is suitable for methods and apparatuses ofexposure wherein the emission cycle is relatively long.

While preferred embodiments of the invention have been described usingspecific terms, such description is for illustrative purposes only andit is to be understood that changes and modifications may be madewithout departing from the spirit or scope of the present invention.Such changes and modifications are intended to be covered by the claims.

What is claimed is:
 1. An exposure method for use in an image forming apparatus having a large number of edge emission type electroluminescent devices, the horizontal width of each of the edge emission type electroluminescent devices being greater than the vertical thickness thereof, the devices being arranged in the horizontal scanning direction and positioned opposite to a photosensitive body so that as the photosensitive body rotates, one edge emission type electroluminescent device emits light a plurality of times onto the surface of the photosensitive body to form one pixel of an electrostatic latent image, the exposure method comprising the steps of:determining one of a first and a second case of print data, the first case being one in which the gradient of the outline of the electrostatic latent image relative to the horizontal scanning direction falls between 0° and 45°, the second case being one in which the gradient of the outline relative to the horizontal scanning direction falls between 45° and 90°; causing, in the first case, the edge emission type electroluminescent devices to emit light selectively according to the proportion of image-filled regions in the pixels intersected by the outline; and setting, in the second case, the light intensity of the edge emission type electroluminescent devices according to the proportion of image-filled regions in the pixels intersected by the outline.
 2. An exposure apparatus having a large number of edge emission type electroluminescent devices, the horizontal width of each of the edge emission type electroluminescent devices being greater than the vertical thickness thereof, the devices being arranged in the horizontal scanning direction and positioned opposite to a photosensitive body so that as the photosensitive body rotates, one edge emission type electroluminescent device emits light a plurality of times onto the surface of the photosensitive body to form one pixel of an electrostatic latent image, the apparatus comprising:data determining means for determining one of a first and a second case of print data, the first case being on in which the gradient of the outline of the electrostatic latent image relative to the horizontal scanning direction falls between 0° and 45°, the second case being one in which the gradient of the outline relative to the horizontal scanning direction falls between 45° and 90°; light emission determining means for causing, in the first case, the edge emission type electroluminescent devices to emit light selectively according to the proportion of image-filled regions in the pixels intersected by the outline; and light intensity setting means for setting, in the second case, the light intensity of the edge emission type electroluminescent devices according to the proportion of image-filled regions in the pixels intersected by the outline; wherein the light emission determining means and the light intensity setting means ar connected to drivers which in turn are connected to the edge emission type electroluminescent devices.
 3. An exposure method for use in an image forming apparatus having a large number of edge emission type electroluminescent devices, the horizontal width of each of the edge emission type electroluminescent devices being greater than the vertical thickness thereof, the devices being arranged in the horizontal scanning direction and positioned opposite to a photosensitive body so that a the photosensitive body rotates, one edge emission type electroluminescent device emits light a plurality of times onto the surface of the photosensitive body to form one pixel of an electrostatic latent image, the exposure method comprising the steps of:determining one of a first and a second case of print data, the first case being one in which the gradient of the outline of the electrostatic latent image relative to the horizontal scanning direction falls between 0° and 45°, the second case being one in which the gradient of the outline relative to the horizontal scanning direction falls between 45° and 90°; causing, in the first case, the edge emission type electroluminescent devices to emit light selectively according to the proportion of image-filled regions in the pixels intersected by the outline; and gradually reducing, in the second case, the number of pulses applied to the edge emission type electroluminescent devices according to reductions of image-filled regions in the pixels intersected by the outline, the pulses exceeding a predetermined threshold voltage value.
 4. An exposure apparatus having a large number of edge emission type electroluminescent devices, the horizontal width of each of the edge emission type electroluminescent devices being greater than the vertical thickness thereof, the devices being arranged in the horizontal scanning direction and positioned opposite to a photosensitive body so that as the photosensitive body rotates, one edge emission type electroluminescent device emits light a plurality of times onto the surface of the photosensitive body to form one pixel of an electrostatic latent image, the apparatus comprising:data determining means for determining one of a first and a second case of print data, the first case being one in which the gradient of the outline of the electrostatic latent image relative to the horizontal scanning direction falls between 0° and 45°, the second case being one in which the gradient of the outline relative to the horizontal scanning direction falls between 45° and 90°; light emission determining means for causing, in the first case, the edge emission type electroluminescent devices to emit light selectively according to the proportion of image-filled regions in the pixels intersected by the outline; and pulse count changing means for gradually reducing, in the second case, the number of pulses applied to the edge emission type electroluminescent devices according to reductions of image-filled regions in the pixels intersected by the outline, the pulses exceeding a predetermined threshold voltage value; wherein the light emission determining means and the pulse count changing means are connected to drivers which in turn are connected to the edge emission type electroluminescent devices.
 5. An exposure apparatus for forming an electrostatic latent image on a photosensitive body by exposing the photosensitive body to light in accordance with the print data representing an image defined by an outline, the apparatus comprising:a line head made of a plurality of flat-shaped edge emission devices arranged in line, each of the edge emission devices emitting light a plurality of times to form one pixel of the electrostatic latent image, the light emission being performed at a light intensity level established in keeping with the operation of the photosensitive body, the line head relatively scanning the photosensitive body in the horizontal direction which is in parallel with the photosensitive body while relatively moving in a direction perpendicular to the horizontal direction at the same time; means for outputting a data decision signal by comparing the gradient of the image outline represented by the print data with a predetermined angle; and data converting means for determining the operation of the edge emission devices of the line head in accordance with the data decision signal, the data converting means including, light emission determining means for determining, when the gradient is less than the predetermined angle, the number of times each of the edge emission devices of the line head emits light to form one of the pixels intersected by the outline, light intensity setting means for setting, when the gradient is greater than the predetermined angle, the light intensity of each of the edge emission devices of the line head in emitting light to form one of the pixels intersected by the outline; and means for controlling the operation of each of the edge emission devices of the line head in accordance with the data from the data converting means.
 6. An exposure apparatus for forming an electrostatic latent image on a photosensitive body by exposing the photosensitive body to light in accordance with the print data representing an image defined by an outline, the apparatus comprising:a line head made of a plurality of flat-shaped edge emission devices arranged linearly, each of the edge emission devices emitting light a plurality of times to form one pixel of the electrostatic latent image when supplied with pulses of which the voltage exceeds a predetermined threshold value, the light emission being performed in keeping with the operation of the photosensitive body, the line head relatively scanning the photosensitive body in the horizontal direction which is in parallel with the photosensitive body while relatively moving in a direction perpendicular to the horizontal direction at the same time; means for outputting a data decision signal by comparing the gradient of the image outline represented by the print data with a predetermined angle; and data converting means for determining the operation of the edge emission devices of the line head in accordance with the data decision signal, the data converting means including, light emission determining means for determining, when the gradient is less than the predetermined angle, the number of times each of the edge emission devices of the line head emits light to form one of the pixels intersected by the outline, pulse controlling means for gradually reducing, when the gradient is greater than the predetermined angle, the number of the pulses applied to each of the edge emission devices of the line head to form one of the pixels intersected by the outline; and means for controlling the operation of each of the edge emission devices of the line head in accordance with the data from the data converting means. 